1. Field of the Invention
The present invention relates to an image processing method and an image processing apparatus. More particularly, in the fields of laser beam printer and printing plate making apparatus, the present invention relates to an image processing method for putting input image data represented by multi-value pixels into gradation representation by binary data of plural minute pixels while being subjected to error diffusion processing, and an image processing apparatus for executing the processing method.
2. Description of the Related Art
In the fields of laser beam printer, ink jet printer, and printing plate making apparatus, an area gradation method and stochastic half toning technology are known as methods for reproducing the density gradation of color-separated multi-value image data. The former represents density gradation by the magnitude of collections (clusters) of binary dots, and the latter visually represents density gradation by the roughness and fineness of binary dots.
As the area gradation method of the former, a dither matrix (concentrated dither) method is known. According to this method, with regular grid points as base points, dot areas are modulated while minute dots are being colored according to an image density. On the other hand, as the stochastic half toning technology of the latter, an error diffusion method is known. According to this method, while each multi-value pixel is binarized by predetermined threshold values, binarization errors are diffused to unprocessed adjacent multi-value pixels to macroscopically preserve density information (non-patent document 1, for example).
Since the dither matrix method performs binarization processing while comparing predetermined threshold values and input image data on a multi-value pixel basis, satisfaction of both gradation properties and resolution requires sufficiently high resolution of the binarization processing and a large size of dither matrix. Regular binary dot arrangement poses the problems of color moire of secondary and tertiary colors and document moire.
On the other hand, the error diffusion method, in which binarized threshold values do not rely on multi-value pixel positions unlike dither matrix, has the characteristics of easy adaptation to an image structure of input image data because of low periodicity of dot structure and macroscopic preservation of density information, and satisfies both gradation properties and resolution and effectively prevents the occurrence of moire pattern.
However, the error diffusion method has had problems in that, if specific data is continuously inputted, diffusion errors have periodicity, as a result of which specific textures occur or stripe-like noise called worm noise mingles. These problems have been conventionally solved by using technology for adding random number data to diffusion errors (patent document 1, for example), and technology for switching factors to be multiplied by diffusion errors in various ways (patent document 2, for example).
FIG. 20 is a schematic diagram showing a binarized image produced by binary error diffusion processing according to the related art of the former. FIG. 21 is a schematic diagram showing a binarized image produced when, with a 2-by-2 bitmap allocated to a multi-value pixel, binary error diffusion processing, that is, high-resolution error diffusion processing is performed on a minute pixel basis. Particularly in an ink jet printer, high quality images can be produced using such a high-resolution error diffusion processing method.
Laser printers employing electrophotographic processes have the restriction of slow-scan resolution of scan beam and therefore a method for achieving high resolution only in a fast-scan direction has been conventionally used (non-patent document 2, for example). In such a recording system, multi-value error diffusion as shown in FIG. 22 can be used. According to the multi-value error diffusion method, one multi-value pixel is equally divided by eight so that it can be represented by nine values. Specifically, each multi-value pixel is made dense by n/8 according to density thereof and an error from an input value is diffused to peripheral pixels, where n is 0, 1, 2, . . . .
In this connection, although the error diffusion method is frequently used in ink jet printers and satisfactorily provides its gradation reproduction effects, in actual situation, it is not so frequently used in later printers in which the dither matrix method and the analog line screen method are dominant (patent documents 3 and 4, for example).
This is because since spatial frequency responses as well as MTF (Modulation Transfer Function) of photosensitive materials deteriorate in electrophotographic processes such as exposure, development, transfer, and fixing, even if an image structure arranged finely with minute dots as shown in FIGS. 21 and 22 is inputted as a recording signal, reproducibility becomes unstable and washout and blocks occur, resulting in the lack of satisfactory gradation representation.
On the other hand, since the dither matrix method and the analog line screen method expand dots at proper locations according to density, stresses on spatial frequencies do not change greatly in density areas, and the number of lines of dots may be set according to the responsibility of printer, so that gradation can be represented by changes in dot area. Therefore, the dither matrix method and the analog line screen method presuppose dot processing in an output part and perform pre-processing such as spatial frequency filter processing for image data to avoid interference with the dot processing.
Technology taking advantage of the characteristics of dot area modulation and the characteristics of error diffusion processing is proposed (patent documents 5 and 6, for example).
According to the technology disclosed in patent document 5, if correction image data (input image data containing added error values) is less than a value corresponding to a reproducible minimum dot size, dots are not generated and an error value is diffused to the periphery, and if the correction image data is equal to or greater than the value corresponding to the reproducible minimum dot size, a dot of a size corresponding to the correction image data is formed and error data is zeroed.
The following technique is disclosed in patent document 6. According to the technique, to prevent washout and blocks, the sizes of dots created by error diffusion processing are subjected to pulse width modulation by density around a multi-value target pixel so that low densities are widely modulated and high densities are narrowly modulated.
[Patent document 1]
Japanese Published Examined Patent Application No. Hei 1-058915
[Patent document 2]
Japanese Published Examined Patent Application No. Hei 6-66873
[Patent document 3]
Japanese Patent No. 2532398
[Patent document 4]
Japanese Published Unexamined Patent Application No. Sho 55-19201
[Patent document 5]
Japanese Patent No. 2664173
[Patent document 6]
Japanese Published Unexamined Patent Application No. 2000-138829
[Non-patent document 1]
R.FLOYD&L.STEINBERG, “An Adaptive Algorithm for Spatial Grey Scale”, SID 75 DIGEST, PP 36-37
[Non-patent document 2]
The Institute of Image Electronics Engineers of Japan draft Jul. 2, 1985, Tomoaki Tanaka et al., “Electronic Photo Recording by Use of Elliptic Beams”
Generally, error diffusion processing is characterized by a dispersed dot structure and is technology that cannot be used without performing processing so that a uniform structure does not appear for a specific image density as described previously. If this processing is insufficient, dispersed structures are interspersed with regular structures, making texture change conspicuous and making the image ugly. In this connection, according to the related art disclosed in patent document 5, error diffusion processing is performed only in highlight portions, and in portions equal to or greater than a specific density, error diffusion processing is not performed and PWM (pulse width modulation) is performed on a multi-value pixel basis, as a result of which an image of line screen structure is produced. Accordingly, an image rich in gradation change, when outputted, becomes ugly because structure change is conspicuous in the vicinity of switching areas.
The error diffusion processing preserves densities, while, in the technology disclosed in patent document 6, since dot sizes are modulated after processing, areas in which density information is not preserved come into existence, with the result that mismatch occurs between diffused density errors and a reproduced image. As a result, gradation jump and density reversal may occur in boundaries between portions subjected to pulse width modulation and portions not subjected to pulse width modulation (PWM).